Method and apparatus for wireless communication based on frequency selective transmission in wireless local area network

ABSTRACT

Disclosed are a wireless communication method, and an access point and a station which perform the wireless communication method. A wireless communication method performed by an access point according to an embodiment may include performing channel sounding on a plurality of subchannels, identifying subchannels selected by stations among the subchannels, scheduling communications between the AP and the stations based on the selected subchannels, and transmitting a data frame to the stations through the subchannels based on a scheduling result.

TECHNICAL FIELD

Exemplary embodiments relate to a wireless communication technologybased on frequency selective transmission in a wireless local areanetwork (WLAN).

BACKGROUND ART

A local area network (LAN) as a communication network for a limited areagenerally includes a cabled LAN and a wireless LAN (WLAN). A WLANenables communications via a network using radio waves instead ofemploying a cable. The WLAN has been introduced as an alternative foraddressing difficulties in installation, maintenance and movement causedby cabling and become increasingly needed with growing mobile users.

A WLAN is formed of an access point (AP) and a station (STA) as aterminal device. The AP is a device sending radio waves to enable WLANusers within in a coverage area to connect to the Internet and to usethe network, which functions like a hub of a cabled network. An AP for awireless high-speed Internet service provided by an Internet serviceprovider (ISP) is already installed in a service area.

A basic architecture of an IEEE 802.11 network is a basic service set(BSS). The IEEE 802.11 network includes an independent BSS in whichterminals in the BSS communicate directly with each other, aninfrastructure BSS which involves an AP in communications of a terminalwith terminals in and outside the BSS, and an extended service set inwhich BSSs are connected to extend a service area.

DISCLOSURE OF INVENTION Technical solutions

A wireless communication method performed by an access point (AP)according to an embodiment may include performing channel sounding on aplurality of subchannels; identifying subchannels selected by stationsamong the subchannels; scheduling communications between the AP and thestations based on the selected subchannels; and transmitting a dataframe to the stations through the subchannels based on a schedulingresult.

The scheduling of the communications between the AP and the stations mayinclude scheduling a group of stations to which the data frame issimultaneously transmitted through different subchannels based on theselected subchannels.

The transmitting of the data frame may simultaneously transmit the dataframe to the stations through different subchannels based on thescheduling result.

The identifying of the subchannels selected by the stations may identifythe subchannels selected by the stations based on subchannels throughwhich a frame is transmitted from the stations.

The performing of the channel sounding may include transmitting asounding frame for channel sounding through the subchannels.

The transmitting of the sounding frame may transmit a Null Data Packet(NDP) frame through the subchannels within a time interval of aRestricted Access Window (RAW) set by the AP.

The NDP frame may be transmitted with a Long Training Field (LTF) paddedso that NAP frames transmitted through the subchannels have the samelength.

The stations may perform channel estimation based on the NDP frametransmitted by the AP and select a subchannel to use for communicationsamong the plurality of subchannels based on a channel estimation result.

The stations may transmit a feedback on channel information andsubchannel selection information to the AP after receiving the soundingframe.

The transmitting of the data frame may simultaneously transmit dataframes to the stations through antennas corresponding to the respectivesubchannels based on a scheduling result.

The identifying of the subchannels selected by the stations may includereceiving subchannel selection information from the stations; andidentifying the subchannels selected by the stations based on thereceived subchannel selection information, and the subchannel selectioninformation may include selection information on at least one subchannelpreferred by a station among the plurality of subchannels.

The stations may transmit a frame to the AP through the selectedsubchannels among the plurality of subchannels.

The stations may transmit information on at least one or more preferredsubchannels among the plurality of subchannels to the AP.

The transmitting of the data frame may include identifying allmulti-subchannels as one group identification (ID) to conduct signaling.

The transmitting of the data frame may include selecting a single-usermultiple-input multiple-output (SU-MIMO) mode or multi-usermultiple-input multiple-output (MU-MIMO) for each subchannel; andidentifying a subchannel with the SU-MIMO mode selected as anassociation ID (AID) to conduct signaling and identifying a subchannelwith the MU-MIMO mode selected as a group ID to conduct signaling.

The scheduling of the communications between the AP and the stations mayinclude scheduling a frequency resource used for communications betweenthe AP and the stations based on the subchannels selected by eachstation; and broadcasting information on the scheduled frequencyresource.

The stations may adjust a size of a feedback frame and transmit thefeedback frame with the size adjusted to the AP.

A wireless communication method performed by a station according to anembodiment may include selecting a preferred subchannel among aplurality of subchannels; transmitting a frame to an AP through theselected subchannel; receiving resource scheduling information from theAP; and communicating with the AP based on the resource schedulinginformation.

The selecting of the subchannel may include performing channelestimation based on an NDP frame received from the AP; and selecting thepreferred subchannel among the subchannels based on a channel estimationresult.

An AP according to an embodiment may include at least one antenna; areceiver to receive a frame through a subchannel among a plurality ofsubchannels from each of stations through the at least one antenna; anda transmitter to transmit resource scheduling information based onsubchannels selected by the stations to the stations through the atleast one antenna, wherein the subchannels selected by the stations maybe identified by subchannels used to transmit the frame.

A station according to an embodiment may include at least one antenna; atransmitter to transmit a frame to an AP through a preferred subchannelamong a plurality of subchannels through the at least one antenna; and areceiver to receive resource scheduling information from the AP throughthe at least one antenna, wherein the resource scheduling informationmay be determined based on subchannels selected by stations.

An AP according to an embodiment may include at least one antenna; areceiver to receive feedback frames comprising information on apreferred candidate subchannel among a plurality of subchannels from aplurality of stations through the at least one antenna; and atransmitter to broadcast resource scheduling information generated byscheduling resources between the AP and the stations to the stations,wherein the resource scheduling information may be determined based onthe information on the candidate subchannel.

The AP may schedule frequency resources by allocating subchannels usedfor communications to the stations based on the information on thecandidate subchannel.

The AP may allocate subchannels used for communication to the stationsand control the stations to conduct channel sounding through thesubchannels in parallel.

The transmitter may transmit data to the stations through beamforming insubchannels allocated to the stations.

A station according to an embodiment may include at least one antenna; atransmitter to transmit a feedback frame comprising information on apreferred candidate subchannel among a plurality of subchannels to anAP; and a receiver to receive resource scheduling information from theAP through the at least one antenna, wherein the AP may generate theresource scheduling information based on information on a candidatesubchannel received from a plurality of stations.

The AP may transmit an NDP frame through the subchannels, and thetransmitter may transmit, to the AP, a feedback frame on at least onesubchannel among subchannels used to transmit the NDP frame.

The transmitter may transmit a list of candidate subchannels preferredby the station among the subchannels to the AP.

The transmitter may transmit a signal-to-noise ratio (SNR) of asubchannel or SNR of a group of tones to the AP, and the AP may generatethe resource scheduling information based on the SNR of the subchannelor SNR of the group of tones.

The station may perform channel sounding via a subchannel allocated bythe AP.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless local area network (WLAN) environmentaccording to an embodiment.

FIG. 2 illustrates a configuration of a wireless device adoptable for aWLAN system according to an embodiment.

FIG. 3 illustrates a subchannel selective transmission (SST) operationaccording to an embodiment.

FIG. 4 illustrates a scheduling operation of an access point (AP)according to an embodiment.

FIGS. 5 and 6 illustrate multi-subchannel transmission of a Null DataPacket (NDP) packet for a single-user multiple-input multiple-output(SU-MIMO) mode according to embodiments.

FIGS. 7 and 8 illustrate multi-subchannel transmission of an NDP packetfor the multi-user multiple-input multiple-output (MU-MIMO) modeaccording to embodiments.

FIG. 9 illustrates a scheduling operation of an AP according to anotherembodiment.

FIG. 10 illustrates a format of an NDP sounding packet in IEEE 802.11ahaccording to an embodiment.

FIG. 11 is a flowchart illustrating operations of a wirelesscommunication method performed by an AP according to an embodiment.

FIG. 12 is a flowchart illustrating operations of a wirelesscommunication method performed by a station according to an embodiment.

FIG. 13 is a flowchart illustrating operations of a wirelesscommunication method performed by an AP according to another embodiment.

FIG. 14 is a flowchart illustrating operations of a wirelesscommunication method performed by a station according to anotherembodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. It isto be understood that the detailed description, which will be disclosedalong with the accompanying drawings, is intended to describe theexemplary embodiments of the present invention, and is not intended todescribe a unique embodiment with which the present invention can becarried out. Hereinafter, the following detailed description includesdetailed matters to provide full understanding of the present invention.However, it will be apparent to those skilled in the art that thepresent invention can be carried out without the detailed matters.

The following embodiments are constructed by combining components andfeatures of the present invention into particular forms. Each componentor feature may be considered optional unless mentioned otherwise. Eachcomponent or feature may be embodied in a separate form from anothercomponent or feature. Also, some components and/or features may becombined to construct an embodiment of the present invention. Operationsillustrated in embodiments of the present invention may be carried outin a different order. Some elements or features of one embodiment may beincluded in another embodiment or be replaced with correspondingelements or features of another embodiment.

Specific terms to be used in the following description are provided forbetter understanding of the present invention and may be changed withother forms without departing from the technical scope of the presentinvention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, like reference numeralsrefer to like elements throughout the specification.

The embodiments of the present invention may be supported by standardsdisclosed in at least one of wireless access systems, such as Instituteof Electrical and Electronics Engineers (IEEE) 802 system, 3rdGeneration Partnership Project (3GPP) system, 3GPP Long-Term Evolution(LTE) and LTE-Advanced (LTE-A) system and 3rd Generation PartnershipProject 2 (3GPP2) system. That is, operations or portions not mentionedin the embodiments to clarify the technical scope of the presentinvention may be supported by the standards. All terms used in thisspecification may be explained by the standards.

The following technology may be used for various types of wirelessaccess systems, such as Code Division Multiple Access (CDMA), FrequencyDivision Multiple Access (FDMA), Time Division Multiple Access (TDMA),Orthogonal Frequency Division Multiple Access (OFDMA), and SingleCarrier Frequency Division Multiple Access (SC-FDMA). CDMA may berealized by radio technologies, such as Universal Terrestrial RadioAccess (UTRA) or CDMA2000. TDMA may be realized by radio technologies,such as Global System for Mobile communications (GSM)/General PacketRadio Service (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMAmay be realized by radio technologies, such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). Although thefollowing description will be made with reference to the IEEE 802.11system for clarity, the technical scope of the present invention is notlimited thereto.

FIG. 1 illustrates a multi-input multi-output (MIMO) wireless local areanetwork (WLAN) system according to an embodiment.

The WLAN system 100 may include one or more basic service sets (BSSs).The WLAN system 100 may include an access point (AP) 110 and stations(STAs) 120 a to 120 i. For simplicity, FIG. 1 shows a single AP 110only.

The AP 110 is a functional entity which provides connection to adistributed system 100 via a wireless medium for an STA associated withthe AP 110. The AP 110 may communicate with one or more STAs at arandomly determined moment via downlinks and uplinks of the STAs.Downlinks are communication links from the AP 110 to the STAs 120 a to120 i, and uplinks are communication links from the STAs 120 a to 120 ito the AP 110. An STA may communicate with another STA peer to peer.

In a BSS including the AP 110, communications between STAs are basicallyperformed via the AP 110, but direct communications between STAs arepossible, not via the AP 110, when a direct link is established betweenthe STAs. The AP 110 may also be referred to as and configured as, forexample, a central controller, a base station (BS), node-B or a basetransceiver system (BTS).

An STA may also be referred to as and configured as a mobile terminal, awireless device, a wireless transmit/receive unit (WTRU), user equipment(UE), a mobile station (MS), a mobile subscriber unit or simply a user.

The AP 110 may simultaneously transmit data to a station group includingat least one or more STAs among the STAs 120 a to 120 i associated withthe AP 110.

The wireless system 100 supports multi-user multi-input multi-output(MU-MIMO) communication. The MU-MIMO communication system 100 enablesthe AP 110 to transmit a plurality of spatial streams to a plurality ofSTAs using a multi-antenna. Also, when the AP 110 employs a plurality oftransmitting antennas, the AP 110 may transmit data frames to the STAs120 a to 120 i using beamforming (BF) technology to improve transmissionperformance.

When the AP 110 conducts frequency selective transmission (FST) using anarrowband subchannel in a broadband BSS of the wireless system 100, theAP 110 may simultaneously perform FSTs using different subchannels.

When subchannels to be used by the STAs are identified before data istransmitted to the STAs, the AP 110 may create one group from STAs touse different subchannels and schedule a single transmission for thegroup. For instance, the STAs may transmit information on subchannels touse for FST to the AP 110, and the AP 110 may schedule communicationswith the STAs 120 a to 120 i based on the received information on thesubchannels from the STAs.

The AP110 may simultaneously conduct FSTs through orthogonal subchannelsin MU-MIMO mode, thereby improving throughput of the network as comparedwith FST using a single subchannel.

The AP 110 needs channel information on STAs to which a data frame istransmitted and may perform channel sounding to acquire the neededchannel information. Channel sounding means a procedure of feeding backchannel status information, which may be performed based on a Null DataPacket (NDP) frame and an NDP Announcement (NDPA) frame. An NDP framehas a PLCP Protocol Data Unit (PPDU) format excluding a data field of aMedia Access Control (MAC) layer and may also be referred to as asounding frame. PLCP represents Physical Layer Convergence Procedure.The AP 110 may use an NDP frame to extract the channel information fromthe STAs.

The STAs may perform channel estimation based on the NDP frame receivedfrom the AP 110 and feed channel status information as a channelestimation result back to the AP 110. For example, the STAs may estimatean MIMO channel based on Very High Throughput-Long Training Fields(VHT-LTFs) of the NDP frame to acquire the channel information.

An NDPA frame is transmitted to indicate an STA to receive an NDP frameand may also be referred to as a sounding announcement frame. An STA maydetermine through an NDPA frame whether the STA participates in channelsounding. The AP 110 may include information on a target STA of channelsounding in an NDPA frame to transmit to the STA. The AP 110 may directthe STA to receive an NDP frame using the NDPA frame.

The AP 110 may transmit the NDPA frame and the NDP frame based on aRestricted Access Window (RAW). An RAW is a time period during whichonly specific STAs are allowed to access a channel but other STAs arenot allowed.

The AP 110 uses a plurality of transmitting antennas and a plurality ofreceiving antennas for data transmission via downlinks and uplinks. Eachof the STAs 120 a to 120 i may include one or more antennas. The STAs120 a to 120 i may have the same number or different numbers ofantennas.

According to another embodiment, in a WLAN system 100 supporting OFDMA,the AP 110 may schedule communications of STAs for simultaneoustransmissions via frequency resources with different bandwidths andmatch transmission times and lengths of frames. Accordingly,simultaneous transmissions through synchronization of the frames areenabled in frame exchanges within a Transmit Opportunity (TXOP) obtainedby the AP 110 even in an asynchronous WLAN, thus improving throughput ofthe network.

In subchannel selective transmission (SST) as FST, each of the STAs 120a to 120 i selects one subchannel in each beacon interval and exchangesframes with the AP 110 through the selected subchannel. Here, in onebeacon interval, the STAs 120 a to 120 i are not allowed to move toother subchannels provided by the AP 110 to conduct SST and to transmitframes through a plurality of subchannels. Thus, the STAs may havedifficulty in using an optimal frequency resource depending on trafficor rapid channel change. To employ an optimal frequency resourcedepending on traffic or rapid channel change, OFDMA assigning STAsresources varying on circumstances of the STAs is employed.

Hereinafter, an embodiment in which OFDMA is possible will be described.

Like SST, OFDMA also needs a sounding process in order to indicate achannel favored by an STA to the AP 110. Sounding may be performed bysimultaneously transmitting an NDP packet, the NDP packet beingconstructed in a sequential sweep for channel estimation of an OFDMAunit resource, for example, a frequency and time, being constructed initerative bandwidth mode, or being constructed in combination of thesequential sweep and iterative bandwidth mode. After sounding isperformed using a sounding packet, such as an NDP packet, the STA maytransmit a feedback on channel information and information on a favoredsubchannel to the AP 110. Feeding back all channel information enablesthe AP 110 to conduct optimal resource scheduling but causes too heavyfeedback information, whereas conducting transmission merely via afavored subchannel without sending information on the favored subchannelas in SST minimizes a feedback information amount but allows onlyrestricted scheduling since scheduling is possible only via a subchannelselected by an STA. In the present invention, STAs may provide channelindication to the AP 110 through the foregoing two-operation method.

In a first operation, the STAs 120 a to 120 i transmit information oncandidate subchannels preferred by the STAs to the AP 110. Here, theSTAs 120 a to 120 i may also feed channel status information back to theAP 110. For instance, the STAs may provide an average signal-to-noiseratio (SNR) by subchannel or an SNR of a group of specific tones as thechannel status information to the AP 110.

The STAs 120 a to 120 i may feed subchannels having a metric exhibitinga channel characteristic of a subchannel, for example, an SNR, greaterthan a predetermined threshold as candidate subchannels back to the AP110 according to predetermined rules. Also, when the AP 110 conductsscheduling by a plurality of predetermined adjacent subchannels, a listof bands with channel status information greater than a threshold amongbands including the plurality of adjacent subchannels may be fed back tothe AP 110. Here, to apply frequency selectivity as an advantage ofOFDMA, resources may be classified into groups of a plurality of smallertones other than a single subchannel, and a list of the resources andchannel information by resource may be fed back. The resources may beinterleaved resources formed by grouping a plurality of interspersedtones in a frequency space, or be burst resources formed by groupingadjacent tones. Unlike SST indication, frame transmission of an STA forsuch indication at 2.4 GHz or 5 GHz is basically carried out through aprimary channel.

The AP 110 collecting indication information on the STAs 120 a to 120 imay schedule resources to allocate to the STAs 120 a to 120 i based onthe collected indication information. Here, the AP 110 may schedulefrequency resources of the STAs. Scheduling information may include alist of STAs to simultaneously use a divided frequency resource andinformation on a resource to be used by each STA. The schedulinginformation may be constructed as an STA identification (ID) list bysequentially allocating unit frequencies to STAs, or as an index list bydetermining an OFDMA group in advance like an MU-MIMO group,transmitting an OFDMA group ID only, and allocating an index of an STAin the OFDMA group for resource allocation. In this case, an STA ID is 6bytes in an MAC address, and 2 bytes even in use of an association ID(AID), and STAs are identified by bits expressing a number of STAsbelonging to a group, and thus pieces of information as many as bits *unit resource may be provided. Also, each frequency resource may beallocated not to a single STA but to a plurality of STAs as in anMU-MIMO group. The AP 110 broadcast the scheduling information on thefrequency resource, so that the STAs 120 a to 120 i may identify whetherthe STAs 120 a to 120 i are included in a schedule and identify alocation of a frequency resource to use if included. Further,broadcasting the scheduling information is basically carried out bytransmission including a primary channel and may be conducted induplicate (DUP) mode for protection against other channels.

In a second operation, the AP 110 may perform accurate channelestimation to improve throughput by scheduled frequency resource.Channel estimation is performed by frequency resource and aims atsimultaneously estimating a plurality of resources. Channel estimationis similar to SU/MU BF sounding performed by subchannel after SSTindication mentioned above. A difference is that a bandwidth for SU/MUsounding may vary by frequency resource in OFDMA, unlike a fixedsubchannel. For instance, when a 50 GHz band is used, one frequencyresource is 20 MHz or another frequency resource is 60 MHz. Here, sinceBF feedback may be performed by subcarrier to cause a substantial amountof information, feedback via other resources may be performed inparallel to reduce necessary time for feedback. Another difference isthat OFDMA may perform transmit power control to decrease interference,for which necessary information may be exchanged. To this end, the AP110 may include power information in a scheduling announcement frame,and the STAs may transmit a feedback including power relatedinformation. Information to be included may vary on a transmit powercontrol algorithm.

In addition to BF feedback, matching sizes of feedback frames of theSTAs to simultaneously conduct feedback is also essentially considered.Lengths of the feedback frames vary depending on quantities for the STAsto feed back and a modulation and coding scheme (MCS) selected atfeedback. However, in order that the AP 110 having a single modelconducts simultaneous transmission/reception and switchestransmission/reception modes, not only times at which the STAs sendframes but also lengths of the frames need matching. Then, after theframes are received, the transmission/reception modes are switched tosimultaneously transmit a BF report poll frame for requesting a nextfeedback with respect to a plurality of resources after a ShortInterframe Space (SIFS). To this end, the AP 110 may additionallyannounce a duration value of one packet via the scheduling announcementframe, the NDP announcement frame or the BF report poll frame. When a BFfeedback frame of an STA is longer than the duration value, the STA maytransmit the BF feedback frame via fragmentation according to a BFsounding protocol. When the poll frame is transmitted from the AP 110,the STA may transmit a subsequent segment to the AP 110. However, whenthe BF feedback frame is shorter than the duration value announced bythe AP 110, feedback data is transmitted as an Aggregated MAC ProtocolData Unit (A-MPDU), and accordingly the AP 110 may transmit the A-MPDUvia padding to correspond to the duration value. When the BF feedbackframe is matched, the AP 110 is able to match transmission times of pollframes and also match lengths of frames to be simultaneously transmittedby the AP 110, and thus frame exchanges is carried out within an SIFS ina TXOP for sounding of a plurality of resources simultaneouslyestablished without interference by other STAs.

When duration information is added in BF sounding, the durationinformation may also be used for multi-subchannel SU/MU-MIMO soundingusing SST and data transmission. Here, to announce a new duration value,the AP 110 may define and announce an existing NDPA frame or newAnnouncement frame so that the STAs match durations of uplink frames anddo not extend a bandwidth beyond subchannels thereof to conducttransmission since other subchannels are used for transmission.

When the aforementioned two operations of sounding are finished, the AP110 may start data transmission. In this case, data transmission may beconducted following sounding in the same TXOP as for sounding or beconducted in a new TXOP. Through sounding, all time resources and powerresources by STAs may be allocated. STAs participating in actual OFDMAdata exchanges may vary considering BF sounding and power control, inwhich case new frequency resources may be allocated. Resource schedulinginformation regarding frequency, time and power may be broadcast throughan OFDMA indication frame broadcast by the AP.

The STAs may receive the OFDMA indication frame and identify from theOFDMA indication frame whether the STAs are included in a schedule,durations and locations of frequency resources allocated to the STAs.Also, the STAs may identify a transmit power level for a frame that theAP 110 transmits via a downlink, and accordingly a clear channelassessment (CCA) level may be adjusted in a corresponding frequencyresource. After an SIFS since transmission of the indication frame, theAP 110 may transmit data through the frequency resources, and transmitdata through BF when SU/MU-MIMO BF sounding is performed by resource.The data may be simultaneously transmitted through the frequencyresources, and durations of downlink packets are the same to matchtransmission times of subsequent uplink packets of the STAs, and theSTAs 120 a to 120 i may transmit response frames or data frames to theframes received by the STAs to the AP 110. Here, the STAs may transmitdata at a transmit power level set by the AP 110.

FIG. 2 illustrates a configuration of a wireless device adoptable for aWLAN system according to an embodiment.

The wireless device 200 is an illustrative apparatus configured toimplement various methods described herein. The wireless device 200 maybe an AP or STA according to the present invention.

The wireless device 200 includes a processor 210 controlling anoperation of the wireless device 200. The processor 210 may also bereferred to as a central processing unit (CPU). A memory 220 may provideprogram instructions and data to the processor 210 and include both aread only memory (ROM) and a random access memory (RAM). The processor210 generally performs logical and arithmetic operations based on theprogram instructions stored in the memory 220. Furthermore, theprocessor 210 may detect and quantify levels of signals received by atransceiver 230. The program instructions stored in the memory 220 maybe executable to implement the methods illustrated herein.

The wireless device 200 may include the transceiver 230 to communicatewith other devices. The transceiver 230 may include a transmitter 240and a receiver 250 and be controlled by the processor 210. The wirelessdevice 200 may include one antenna or a plurality of antennas, and theantennas may be electrically coupled to the transceiver 230.

The wireless device 200 may detect and quantify the levels of thesignals received by the transceiver 230. The wireless device 200 maydetect the signals as total energy, energy per sub-carrier by symbol,power spectral density and other signals. The processor 210 may digitizesignals to process digital signals.

In one embodiment, when the wireless device 200 operates as an AP, thereceiver 240 may receive a frame through any one of a plurality ofsubchannels from STAs through at least one antenna. The transmitter 250may transmit resource scheduling information based on subchannelsselected by the respective STAs to the STAs through the at least oneantenna. The subchannels selected by the STAs may be identified throughsubchannels through which frames of the STAs are transmitted.

In one embodiment, when the wireless device 200 operates as an STA, thetransmitter 250 may transmit a frame to an AP through a preferredsubchannel among a plurality of subchannels through at least oneantenna. The AP may sequentially or simultaneously transmit an NDP framethrough the subchannels, and the STA may receive the NDP frame andconduct channel estimation on the subchannels based on the NDP frame.The STA may determine a subchannel preferred by the STA based on achannel estimation result and transmit a frame through the preferredsubchannel, thereby notifying the AP of information on the preferredsubchannel. The receiver 240 may receive resource scheduling informationfrom the AP through the at least one antenna. The resource schedulinginformation may be determined based on a subchannel selected by eachSTA.

In another embodiment, when the wireless device 200 operates as an AP inan OFDMA communication system, the receiver 240 may receive feedbackframes including information on a preferred candidate subchannel among aplurality of subchannels from a plurality of STAs through at least oneantenna. The AP may schedule subchannels to allocate to the STAs basedon the feedback frames received from the STAs. The AP may determinesubchannels to allocate to the respective STAs based on the informationon the candidate subchannel received from the STAs and generate resourcescheduling information including information on the subchannelsallocated to the STAs. The AP may generate resource schedulinginformation on frequency resources based on channel status informationand the information on the candidate subchannel received from the STAs.The transmitter 250 may broadcast the resource scheduling informationgenerated by scheduling resources between the AP and the STAs to theSTAs. The AP may control the STAs to adjust transmission times and sizesof the feedback frames received from the STAs. The transmitter 250 maytransmit packet duration information for matching the sizes of thefeedback frames to the STAs. The STAs may adjust the sizes of thefeedback frames based on the packet duration information received fromthe AP. The packet duration information may be included in any one of ascheduling announcement frame, an NDPA frame and a BF report poll frameat transmission. The transmitter 250 may simultaneously transmit data tothe plurality of STAs through the plurality of subchannels based on theresource scheduling information. The transmitter 250 may transmit datato the STAs through BF via the subchannels allocated to the STAs. Theforegoing process may improve throughout of the network throughmulti-user diversity by utilizing frequency selectivity.

Alternatively, when the wireless device 200 operates as an STA in theOFDMA communication system, the STA may perform channel estimation basedon a sounding packet received from the AP. The transmitter 250 maytransmit coarse information needed for resource scheduling performed bythe AP to the AP. For example, the transmitter 250 may transmit a listof preferred candidate subchannels among a plurality of subchannels, SNRinformation on the subchannels or SNR information by tone group to theAP. Specifically, the transmitter 250 may transmit channel statusinformation such as the SNR information on the subchannels to the AP.The receiver 240 may receive packet duration information for adjusting asize of a feedback frame from the AP, and the STA may adjust the size ofthe feedback frame based on the packet duration information. Thetransmitter 250 may transmit the feedback frame with the size adjustedbased on the packet duration information back to the AP. The STA mayperform channel sounding only in a subchannel allocated by the AP. Here,the AP may control STAs to conduct channel sounding in parallel throughthe respective subchannels. As channel sounding is conducted inparallel, channel sounding overhead may be reduced. The receiver 240 mayreceive resource scheduling information from the AP through at least oneantenna. The AP may generate the resource scheduling information basedon information on the candidate subchannels received from the pluralityof STAs. The STA may identify a subchannel to use for communication withthe AP based on the resource scheduling information received from theAP, and the transmitter 250 may transmit data to the AP through theidentified subchannel. The STA may transmit data to the AP at atransmission power level determined by the AP.

FIG. 3 illustrates an SST sounding process according to an embodiment.

An AP may communicate with STAs based on FST. In FST, the STAs mayselect optimal frequency bands for the STAs and communicate with the APusing the selected frequency bands. The AP may communicate with the STAsbased on subchannel selective transmission (SST).

The AP may transmit a beacon signal including a channel list enablingSST to the STAs. Information for FST is included in the beacon signal asan SST information element (IE). One SST IE represents channels havingthe same schedule in a channel activity bitmap. The STAs may receive thebeacon signal from the AP and identify channels permitted by the APthrough the SST IE of the beacon signal.

The STAs are allowed to conduct SST through channels to be used by theSTAs among the permitted channels during a period from an Activity StartTime to a Target Beacon Transmission Time (TBTT) at which a next beaconsignal is transmitted and thus may carry out SST during the period.Here, transmission of a frame having a bandwidth (BW) of a maximumpresentation protocol data unit (PPDU) is permitted.

The STAs may transmit data to the AP through the subchannels selected bythe STAs. When the SST IE indicates that an uplink (UL) activity is offand a downlink (DL) activity is on, the STAs may transmit a Power SavePoll (PS Poll) frame 310 including information on the subchannelsselected by the STAs to the AP.

When the AP receives the data from the STAs through the subchannels, theAP may verify that the STAs use the subchannels.

The AP may transmit a downlink frame to the STAs using the subchannelsselected by the STAs up to a maximum transmission width. The AP maysimultaneously transmit data to the STAs conducting SST throughdifferent subchannels.

In downlink transmission that the AP transmits data to the STAs, the APmay transmit frames using different subchannels and different antennas.

The STAs successfully receiving the frames from the AP may transmit anacknowledgement (ACK) frame 320 to the AP at determined times for therespective STAs. When a Block Acknowledgement Request (BAR) frame isreceived from the AP, the STAs may transmit a Block Acknowledgement (BA)frame to the AP.

Since the frames are simultaneously transmitted using the differentsubchannels, throughput increases by about a number of subchannels timeson average as compared with when using one subchannel.

The AP may perform subchannel sounding and sounding for selecting anoptimal subchannel for an STA capable of conducting SST based on a valueof an Active Start Time field of the SST IE after a time at which FST ispermitted.

The AP sets an RAW to allow only an STA participating in SST sounding toaccess a channel and sets an SST sounding RAW and an SST PS-Poll RAW foran STA capable of conducting SST. In a time period set as an RAW, onlyan STA permitted by the AP may communicate with the AP and other STAsnot permitted are prevented from communicating.

The SST sounding RAW is an RAW during which the AP broadcasts a framefor SST sounding.

In the SST PS-Poll RAW, the STA receiving the beacon signal or TrafficIndication Map (TIM) broadcast frame sends a PS-Poll frame through asubchannel to be used by the STA so as to notify the AP that the STA isready to receive data, and thus the AP successfully receiving thePS-Poll frame may identify which subchannel the STA is to use.

The AP may perform SST sounding to acquire information on a subchannelused for SST.

In the SST sounding RAW for SST sounding, the AP may sequentiallytransmit an NDP frame through the subchannels for STAs capable oftransmitting and receiving data only through one subchannel. An NDPframe may have a PPDU format excluding a data field of an MAC layer.

The STA may move the subchannels and estimate a channel through the NDPframe received from the AP in the SST sounding RAW. The STA may selectan optimal subchannel based on a channel estimation result and accessthe channel to transmit a PS-Poll frame to the AP in the SST PS-Poll RAWindicating the selected sub-channel. Subsequently, the STA may performSST through the sub-channel used to transmit the PS-Poll frame afterreceiving an ACK frame from the AP.

FIG. 3 shows an example of the foregoing SST sounding process.Subchannels are scanned and transmitted in the SST sounding RAW, whichis for STAs capable of listening only through a narrowband at a time toacquire channel status information on all subchannels.

However, in the presence of wideband STAs only, a frame for sounding maybe duplicated and transmitted by SST unit in iterative bandwidth mode.Further, the same effect may be obtained by transmitting a beacon signalin iteration mode without a separate RAW. Also, an SST PS-Poll RAW is amethod of restricting channel access to reduce collision probabilitywhen a large number of STAs are present. The PS-Poll frame 310 is forretrieving buffered downlink data from the AP but may be replaced withanother frame, not being necessarily protected by an RAW.

When it is not identified which sub-channel the AP is present in ortransmission needs to be carried out before an Activity Start Time,transmission may also be carried out through a primary channel, in whicha frame format indicating a subchannel to use or field element format isdefined newly. Moreover, to indicate a plurality of subchannels, the STAmay transmit an entire bitmap to the AP, activating a bit correspondingto a preferred subchannel and inactivating a bit corresponding to anon-preferred subchannel in a bitmap representing all subchannels as ina Channel Activity Bitmap field. For example, the STA may set the bitcorresponding to the preferred subchannel to 1 and the bit correspondingto the non-preferred subchannel to 0. The bitmap provides an option tothe AP so that the AP may dynamically set and allocate resources forSTAs in an activated subchannel depending on resources or data amount ina network.

The AP may determine an MU-MIMO group based on the subchannel indicatedby the STA. Here, the subchannel to be used by the STA refers to asubchannel used to transmit the PS-Poll frame 310 of the STA received inthe SST PS-Poll RAW as in FIG. 3. Alternatively, when any PS-Poll frameis not transmitted, the subchannel is a last subchannel used to transmita frame through SST. The AP may calculate time needed for datatransmission based on data amounts to be transmitted to STAs and channelstatus information estimated from indications of the STAs. The AP mayschedule a group of STAs engaged in simultaneous transmissions based ondata amounts, time needed for data transmission, priority informationsuch as an access class category of data, a quality of service parametersuch as latency requirements, scheduling policies, or the like.

The STA may select an optimal subchannel for the STA among a pluralityof narrowband subchannels permitted by the AP. FSTs between the AP andthe STAs using different subchannels are orthogonal, and thussimultaneous data transmissions are possible. When subchannels are used,the subchannels are orthogonal in a frequency domain, enabling anMU-MIMO mode.

FIG. 4 illustrates a scheduling operation of an AP according to anembodiment.

In SST according to the embodiment, the AP and STAs operate as follows.The AP may signal data transmission to an STA group engaged insimultaneous transmission. Signaling schemes include a method ofsignaling to a single group of all multi-subchannels and a method ofsignaling to each subchannel group. The former method is an MU-MIMOextending scheme, in which an entire STA group is identified by a groupID. In the latter method, an SU-MIMO mode or MU-MIMO mode is selected bysubchannel and signaling is conducted by identifying a subchannel withan AID when the SU-MIMO mode is selected or by identifying a subchannelwith a group ID when the MU-MIMO mode is selected. While SST isperformed, the STAs exchange frames only via subchannels selected by theSTAs and operate independently of operations of STAs through othersubchannels. Here, as it is assumed that the AP is a single modem, theAP may need to synchronize frames sent via downlinks inmulti-subchannels and to match lengths of the frames transmitted throughthe respective subchannels. Then, the AP may receive response framestransmitted by the STAs after an SIFS through the respective subchannelswithin a cyclic prefix (CP).

The MU-MIMO extending method will be described. The MU-MIMO modeinvolves a BF sounding process, which may be performed even inmulti-subchannel transmission, because different numbers of antennas areused by the AP per subchannel for SST sounding and for the MU-MIMO modeusing SST. That is, SST sounding uses all antennas of the AP, while theMU-MIMO mode employs different antennas to transmit space time streamsto STAS for channel orthogonality, so that STAs using differentsubchannels obviously use different antennas. Thus, MU-MIMO BF soundingusing an antenna subset needs to be performed by subchannel of theMU-MIMO mode. An MU-MIMO BF sounding RAW of FIG. 4 is an RAW for MU-MIMOBF sounding for SST, which includes NDPA, NDP and BF feedback in order.An NDPA frame may be broadcast in the same format as in the IEEE802.11ac MU-MIMO mode. Here, since STAs use SST, the NDPA frame needs abandwidth iteration mode by SST unit and may be transmitted in non-highthroughput (HT) duplicate mode of IEEE 802.11ac. An NDP frame istransmitted for sounding after an SIFS.

The NDP frame may be transmitted in non-HT duplicate mode the same asthe NDPA frame. An STA listening to the NDPA and NDP frames via asubchannel selected by the STA may transmit a BF feedback (FB) throughthe subchannel as in FIG. 4. Subsequently, the AP may transmit a dataframe through BF. Like a sounding RAW, an RAW for data transmission mayalso be set using IEEE 802.11ah. Here, a slot for each data transmissionmay be allocated in the RAW, wherein a slot for MU-MIMO modetransmission may use a group ID, instead of an AID for identifying anindividual STA.

As in FIG. 4, when a resource allocation (RA) frame is used at abeginning of a data RAW, the AP may signal a slot allocated in the RAWto an STA in an MU-MIMO group through the group ID instead of the AID.Here, a channel indication bitmap field of an

RAW parameter set information element (RPS IE) signaling the RAW needsto set all subchannels to be used by the MU-MIMO group to 1. Further,the RA frame duplicates and transmits a packet transmitted through eachsubchannel by subchannel using the bandwidth iteration mode. That is,when a single subchannel bandwidth is a basic unit of the duplicatemode, the AP may duplicate and transmit a PPDU with a preamble and apayload forming a packet which are formed of the same signals bysubchannel selected by an STA belonging to the MU-MIMO group. However, adata frame transmitted in a next allocated slot is not transmitted inthe bandwidth iteration mode, unlike the broadcast RA frame, becausedifferent antennas correspond to subchannels and different numbers ofantennas are used for the subchannels and thus the preamble changes andeach data is transmitted by STA in unicast mode.

In detail, a common part of a preamble of a data frame transmitted inMU-MIMO mode is common to all MU-MIMO STAs and thus the same common partis transmitted through each subchannel, whereas a different dedicatedpart of the preamble for a stream using a different antenna by STA istransmitted by subchannel. However, a preamble length and a data packetlength may vary due to the different part by STA, and thus padding isperformed to a length of a longest MPDU after a last MPDU in order tomatch the lengths in the MU-MIMO mode of 802.11ac. In the presentinvention, padding is performed in the same manner. Subsequently, ablock acknowledgement (BA) process is also carried out in the same wayas in the MU-MIMO mode of 802.11 ac. Here, a BAR frame and a BA frameare basically transmitted only via the subchannels of the STAs but mayalso be transmitted in bandwidth iteration mode to prevent access byother STAs between the data frame and the BAR frame. FIG. 4 illustratesan example of the aforementioned MU-MIMO BF sounding and SST MU-MIMOdata transmission processes. As shown in FIG. 4, although transmittedthrough different subchannels, BF feedback frames when sounding isperformed or BA frames in data exchanges may be sequentiallytransmitted.

Next, the method of signaling to each subchannel group will bedescribed. There are three types of modes, such as an SU-MIMO mode, anMU-MIMO mode and an NO BF mode, depending on a data frame transmissiontype via each subchannel. The foregoing three transmission modes aresimilar in that transmission times and lengths of NDP frames are matchedwhen the AP conducts transmission via multi-subchannel downlinks, butare different in that different padding methods are employed to matchthe transmission times and the lengths of the NDP frames.

First, sounding in transmission to one STA by subchannel in SU-MIMO modewill be described. When SU BF sounding is performed, an STA INFO fieldof an NDPA payload is different by subchannel. However, a length of thefield is the same despite a different value thereof, and thus an NDPAframe is subjected to the same MCS to be simultaneously transmitted viamulti-subchannels. On the contrary, an NDP frame has different numbersof Long Training Fields (LTFs) for channel estimation by streamdepending on a number of streams for which the STA conducts BF. Thus,LTFs for subchannels are padded to a longest LTF so as to match lengthsof NDP frames.

Alternatively, subchannel sounding of FIG. 4 may be applied to an OFDMAcommunication system. Although a subchannel is SU BF in FIG. 4,subchannel sounding of FIG. 4 may be applied to a communication systemin which SU MIMO and OFDMA are combined with subchannels in parallel.

In SST, the SST sounding process of FIG. 3 is needed and a subchannelselected by an STA is determined based on which subchannel the STA usesto transmit a frame. On the contrary, in the OFDMA system, the APdetermines a subchannel to allocate to each STA, and the STA maytransmit coarse feedback information, such as preferred subchannelinformation and channel status information, to the AP through a primarychannel if the STA does not support SST.

FIGS. 5 and 6 illustrate multi-subchannel transmission of an NDP packetfor the SU-MIMO mode according to embodiments.

FIG. 5 illustrates an example of simultaneously transmitting NDP frameswithout padding when three subchannels are present. An NDP frame 510lacks an LTF, and thus one more LTF needs padding for BF and compressedBF feedback frame time.

FIG. 6 illustrates an example of LTF padding. LTF3 610 is added to theNDP frame 510 by LTE padding. Alternatively, when an SU preamble of IEEE802.11ac is used, a Number of Space Time Stream (NSTS) field of a VeryHigh Throughput (VHT)-signal (SIG)-A1 field may be revised to a value ofan NSTS field of a VHT-SIG-A1 field of an SU preamble of a subchannelhaving a largest number of LTFs among multi-subchannels. Then, althoughan unnecessary LTF is added, an STA waiting for an NDP packet via thesubchannel detects the added LTF, but recognizes a number of streamsallocated to the STA in NDPA and thus ignores the unnecessary LTF eventhough an NDP frame with the added LTF is received. Each STA maycalculate a BF vector and transmit a feedback frame to the AP after anSIFS from an end of the NDP frame.

When MU BF sounding is performed, an STA INFO field of an NDPA payloadin NDPA is different by subchannel, and a number of STA INFO fields maybe determined based on a number of grouped STAs. Thus, to conductsimultaneous transmission to the STAs, the same number of STAs needs toparticipate in MU-MIMO by subchannel. As in the SU-MIMO mode, when thesame MCS is used, the INFO fields have the same length with differentvalues, and thus the NDPA frames may be simultaneously transmittedthrough multi-subchannels with lengths thereof matched. A number of LTFsof an NDP frame may be determined based on a total number of streamsMU-transmitted by subchannel. Thus, LTF padding for subchannels isperformed to a subchannel having a largest number of streams, similarlyto the SU-MIMO mode.

FIGS. 7 and 8 illustrate multi-subchannel transmission of an NDP packetfor the MU-MIMO mode according to embodiments.

FIG. 7 illustrates an example of simultaneously transmitting NDP framesfor the MU-MIMO mode without padding when three subchannels are present.When an AP has, for example, eight antennas, the AP conducts soundingfor eight streams, in which a first subchannel has a largest number ofstreams transmitted as an MU-MIMO group which is four and a longest NDPframe 710. Thus, NDP frames 720 and 730 of other two subchannels areshorter and thus may be transmitted with two more LTFs padded,corresponding to the longest NDP frame 710, as if having four streams.For example, as shown in FIG. 8, the NDP frame 720 of FIG. 7 may bepadded with two LTFs, LTF3 and LTF4 810, and the NDP frame 730 of FIG. 7may be padded with two LTFs, LTF3 and LTF4 820.

Alternatively, when an MU preamble of IEEE 802.11ac is used, a sum offour fields, NSTS [0] to [3] fields, of VHT-SIG-A1 of the MU preamble isa total number of LTFs, and thus numbers of VHT-LTFs of MU preambles ofsubchannels are increased to a greatest sum of the NSTS [0] to [3]fields of a subchannel among all multi-subchannels. When the numbers ofVHT-LTFs are increased, the AP may allocate additional LTFs to STAsallocated streams in an MU-MIMO group. Thus, the AP may match the totalnumber of the four fields to a greatest sum of LTFs of a subchannel,increasing values of NSTS fields assigned the values other than 0 amongthe NSTS [0] to [3] fields.

An STA may identify a number of streams allocated to the STA and a totalnumber of streams (a sum of streams of an STA INFO field) through an STAINFO field of an NDPA. Also, the STA may identify a number of streamsallocated to the STA and an LTF position through an SIG field of an NDP.Although the number of streams allocated to the STA and the total numberof streams in the NDP increase due to added LTFs as compared with in theNDPA, a greater number of streams than a number defined in the NDPA maybe ignored. Thus, as in the SU-MIMO mode, the STA may identify positionsand number of LTFs needed for sounding from a location of the STA and anNSTS field value of a previous STA through a group ID in an NDP preamblefor MU-MIMO. In addition, the STA may use as many LTFs as a number ofstreams of the STA allocated in the NDPA frame at a corresponding LTFposition for sounding and ignore subsequent LTFs additionally allocatedin the NDP preamble.

When a length of the NDP frame is adjusted corresponding to a maximumnumber of antennas, a first beamformee STA receives in an MU-MIMO groupof subchannels has a similar feedback transmission time after an SIFS,and thus the AP receives a feedback within a CP. When the feedback isnot received within the CP due to a substantial variation in distancebetween the AP and the STA, the AP may need to perform ranging inadvance so that STAs conduct transmission with an offset, not after anSIFS, thereby receiving the feedback within the CP. In a third casewhere an STA receives downlink data without conducting BF, a BF soundingprocess is unnecessary.

When the AP transmits data or an NDPA through SST, content in an SIGfield may vary by subchannel. An AID or group ID for identifying an STAand an NSTS field may be encoded independently by subchannel.

A multi-subchannel transmission method in data transmission aftersounding is carried out as follows. Regardless of performance ofsounding and difference in method between the SU-MIMO mode/MU-MIMO mode,a data frame always includes a payload unlike an NDP frame. Thus, evenwhen data frames have different lengths of preambles, lengths of thedata frames may be adjusted by padding payloads. Data padding ispossible in the MU-MIMO mode of IEEE 802.11 ac, because an aggregatedMAC protocol data unit (AMPDU) is used for transmission. It is assumedthat an AMDPU is employed to use the same padding in the presentinvention. In the MU-MIMO mode, padding is used to match lengths in aPHY layer, and thus padding may be employed in the same manner. When anAMPDU is not supported, the lengths are difficult to match to causedifferent transmission times of response frames such as ACK frames, andthus different transmission times of STAs need to be set by allocatingan offset. In addition, when an RAW is set using the IEEE 802.11ahtechnology and a resource allocation (RA) frame is used at a beginningof a DATA an AID field used in the SU-MIMO mode or in the absence of BFhas the same length as a group ID field in the MU-MIMO mode and thuspayloads have the length.

In the OFDMA communication system, when an AP transmits an OFDMAindication frame and transmits data after an SIFS, the same operation asin SST may be performed. At 2.4 GHz and 5 GHz with legacy, an SIGA fieldis transmitted in common, and thus the AP may divide the SIGA field intoan SIG A field and an SIG B field, transmit the SIG A field includingsubchannel allocation information in common, and subsequently transmitthe SIG B including an AID/group ID by subchannel and NSTS. The SIG Afield may be transmitted in DUP mode so that an STA is able to listenthrough all subchannels. Here, an

OFDMA group including STA information by subchannel is defined in theSIG A field and an OFDMA group ID may be included therein. The STA maydetermine through the SIGA A field whether a subchannel is allocated tothe STA and operate in sleep mode when the subchannel is not allocatedto the STA.

FIG. 9 illustrates a scheduling operation of an AP according to anotherembodiment.

In SST according to one embodiment, the AP and STAs operate as follows.In detail, FIG. 9 shows that the AP schedules packet transmission timesand frame transmission durations to conduct MU-MIMO BF sounding andMU-MIMO data transmission so as to simultaneously conduct a plurality ofSSTs in SST.

To match the transmission durations, both the AP and the STAs need toperform LTF padding on NDP packets and to perform aggregate-MAC protocoldata unit (A-MPDU) padding on a normal A-MPDU packet.

In FIG. 9, ‘P’ represents a BF report poll frame, and ‘BAR’ represents aBlock ACK Request frame. ‘BA’ represents a Block ACK frame, and ‘A’represents an ACK frame. ‘FB’ represents a compressed BF feedback frame,and ‘RA’ represents an RA frame. ‘D1’, ‘D2’ and ‘D3’ represent dataframes transmitted from the AP to the STAs through subchannel 1,subchannel 2 and subchannel 3, respectively.

SST is basically performed through a 2 MHz-unit subchannel but may alsobe conducted via a combination of subchannels. To allow the STAs toselect optimal subchannels, the AP may conduct channel sounding, inwhich regarding channel sounding, there are a parallel mode oftransmitting a sounding packet in the iterative bandwidth mode based ona basic unit of 2 MHz and a series mode of sequentially transmittingpackets by 2 MHz.

When the packets transmitted through the subchannels are received, theSTAs may conduct channel estimation and select subchannels optimal forthe STAs. The STAs may transmit a packet to the AP through the selectedsubchannels to implicitly announce information on the selectedsubchannels. When SU-MIMO BF is needed for the STAs subsequently,SU-MIMO BF sounding is necessarily performed before data transmission.However, since SU-MIMO BF sounding is performed on one STA at a time,decreasing air time is needed. To decrease air time, the AP may conductSST sounding and SU-MIMO BF sounding using NDP packets with the sameformat so as to perform SU-MIMO BF sounding simultaneously with SSTsounding.

Alternatively, subchannel sounding of FIG. 9 may be applied to the OFDMAcommunication system. In this case, FIG. 9 illustrates MU BF bysubchannel in a combination of OFDMA and MU-MIMO. In the OFDMA system,an AP determines subchannels to allocate to STAs, and the STAs maytransmit coarse feedback information, such as preferred subchannelinformation and channel status information, to the AP through a primarychannel if the STAs do not support SST.

FIG. 10 illustrates a format of an NDP sounding packet in IEEE 802.11ahaccording to an embodiment.

It may be identified how many space time streams are used through AnNSTS field of SIG-A1 of the NDP sounding packet. When SST sounding isperformed, an NDP sounding format with a long preamble, not with a shortpreamble having a single LTF only, may be used so that an STA capable ofperforming BF selects a subchannel based on a plurality of antennastream channels. Each STA may select a suitable subchannel for the STAbased on BF capability and space time stream processing ability of theSTA.

FIG. 11 is a flowchart illustrating operations of a wirelesscommunication method performed by an AP according to an embodiment.

In operation 1110, the AP may perform channel sounding on a plurality ofsubchannels. The AP may transmit sounding frames for channel soundingthrough the subchannels. The AP may set an RAW and transmit NDP framesthrough the respective subchannels within a time interval of the RAW.The NDP frames may be transmitted with an LTF padded such that the NDPframes transmitted through the subchannels have the same length.

STAs may perform channel estimation based on the NDP frames transmittedby the AP. The STAs may select a subchannel to use for communicationamong the plurality of subchannels based on a channel estimation result.After the sounding frames are received, the STAs may transmit a feedbackon channel information and subchannel selection information to the AP.

In operation 1120, the AP may identify subchannels selected by the STAsamong the subchannels. The respective STAs may transmit a frame to theAP through the selected subchannels. The AP may identify the subchannelsselected by the STAs based on the subchannels through which the frame istransmitted from the STAs.

Alternatively, the STAs may transmit information on at least one or morepreferred subchannels among the plurality of subchannels to the AP. TheAP may receive the subchannel selection information from the STAs andidentify the subchannels selected by the STAs based on the receivedsubchannel selection information. The subchannel selection informationmay include selection information on at least one subchannel preferredby the STAs among the subchannels.

In operation 1130, the AP may schedule communications between the AP andthe STAs based on the subchannels selected by the STAs. The AP mayschedule a group of STAs to which a data frame is simultaneouslytransmitted through different subchannels based on the subchannelsselected by the STAs. The AP may schedule a frequency resource used forcommunications between the AP and the STAs based on the subchannelsselected by the STAs. Subsequently, the AP may broadcast information onthe scheduled frequency resource. The STAs may adjust a size of afeedback frame and transmit the feedback frame with the adjusted size tothe AP.

In operation 1140, the AP may transmit a data frame to the STAs throughthe subchannels based on a scheduling result of operation 1130. The APmay simultaneously transmit a data frame to the STAs through differentsubchannels based on the scheduling result. The AP may simultaneouslytransmit the data frame to the STAs through antennas corresponding tothe respective subchannels based on the scheduling result.

According to one embodiment, the AP may identify all multi-subchannelsas a single group ID to conduct signaling. Alternatively, the AP mayselect an SU-MIMO mode or MU-MIMO mode for each subchannel. The AP mayidentify a subchannel with the SU-MIMO mode selected as an AID toconduct signaling and identify a subchannel with the MU-MIMO modeselected as a group ID to conduct signaling. Operations of the AP notmentioned in FIG. 11 may refer to descriptions of FIGS. 1 to 10.

FIG. 12 is a flowchart illustrating operations of a wirelesscommunication method performed by an STA according to an embodiment.

In operation 1210, an STA may select a preferred subchannel from aplurality of subchannels. The STA may perform channel estimation basedon an NDP frame received from an AP and select the preferred subchannelfrom the subchannels based on a channel estimation result. The NDP framemay be transmitted through each subchannel within a time interval of anRAW set by the AP. The NDP frame may be transmitted with an LTF beingpadded such that NDP frames transmitted through the subchannels have thesame length. For instance, the STA may perform channel estimation bysubchannel in a channel sounding process performed by the AP and selecta subchannel with a most favorable SNR as the preferred subchannel.

In operation 1220, the STA may transmit a frame to the AP through thesubchannel selected in operation 1210. For instance, the STA maytransmit a PS-Poll frame to the AP. Frame transmission from the STA maybe protected by the RAW set by the AP.

In operation 1230, the STA may receive resource scheduling informationfrom the AP. The AP may schedule a group of STAs to which a data frameis simultaneously transmitted through different subchannels based onsubchannel selected by STAs. The AP may schedule frequency resourcesused for communications between the AP and the STAs based on thesubchannels selected by the STAs. Subsequently, the AP may broadcastresource scheduling information on the scheduled frequency resources.

In operation 1240, the STA may communicate with the AP based on theresource scheduling information. The AP may simultaneously transmit adata frame to the STAs through antennas corresponding to the subchannelsbased on a scheduling result. Operations of the STA not mentioned inFIG. 12 may refer to descriptions of FIGS. 1 to 10.

FIG. 13 is a flowchart illustrating operations of a wirelesscommunication method performed by an AP according to another embodiment.

In operation 1310, the AP may perform channel sounding through asounding packet. For example, the AP may sequentially or simultaneouslytransmit an NDP frame through subchannels. STAs may perform channelestimation on the subchannels based on the received NDP frame anddetermine candidate subchannels preferred by the STAs from the pluralityof subchannels.

In operation 1320, the AP may receive feedback frames includinginformation on the preferred candidate subchannels from the STAs. The APmay allocate each STA a subchannel to use for communication to schedulea frequency resource. The AP may allocate STAs subchannels to use forcommunications and control the STAs to conduct channel sounding inparallel through the allocated subchannels.

The AP may control the STAs to adjust transmission times and sizes ofthe feedback frames received from the STAs. The AP may transmit packetduration information for matching the sizes (or lengths) of the feedbackframes to the STAs. The STAs may adjust the sizes of the feedback framesbased on the packet duration information received from the AP. Thepacket duration information may be included in any one of a schedulingannouncement frame, an NDPA frame and a BF report poll frame attransmission. The AP may adjust the transmission times and sizes of theframes transmitted by the STAs to enable simultaneous transmissions evenin an asynchronous WLAN system, thereby improving throughput of thesystem.

In operation 1330, the AP may broadcast resource scheduling informationdetermine based on the information on the candidate subchannels. The APmay determine subchannels to allocate to the respective STAs based onthe received information on the candidate subchannels and generateresource scheduling information including information on the subchannelsallocated to the STAs. The AP may generate resource schedulinginformation on a frequency resource based on channel status informationand the information on the candidate subchannels received from the STAs.The AP may broadcast the resource scheduling information generated byscheduling frequency resources between the AP and the STAs to the STAs.The STAs may perform channel sounding in the subchannels allocated bythe AP.

In operation 1340, the AP may transmit data to the STAs through thesubchannels based on the resource scheduling information. The AP maysimultaneously transmit the data to the STAs through BF in thesubchannels allocated to the respective STAs.

Operations of the AP and the STAs not mentioned herein may refer torelevant descriptions of FIGS. 1 to 12.

FIG. 14 is a flowchart illustrating operations of a wirelesscommunication method performed by an STA according to anotherembodiment.

In operation 1410, the STA may receive a sounding packet from an AP. TheSTA may perform channel estimation on a subchannel based on the soundingpacket. For instance, the STA may estimate an SNR of the subchannel. TheAP may sequentially or simultaneously transmit an NDP frame throughsubchannels, and the STA may perform channel estimation based on the NDPframe received from the AP.

In operation 1420, the STA may transmit a feedback frame includinginformation on a preferred candidate subchannel of the plurality ofsubchannels to the AP. The STA may transmit a feedback frame on at leastone subchannel among the subchannels used to transmit the NDP frame tothe AP. The STA may select a preferred candidate subchannel among thesubchannels and transmit information on the selected candidatesubchannel through the feedback frame. The STA may transmit a list ofpreferred candidate subchannels among the subchannels to the AP. Here,the STA may adjust a size (or length) of the feedback frame based onpacket duration information received from the AP and may transmit thefeedback frame with the size adjusted back to the AP.

The STA may transmit channel status information along with the feedbackframe to the AP. For instance, the STA may transmit SNR information onthe subchannel or SNR information on a tone group to the AP.

The AP may schedule a frequency resource based on feedback frames andchannel status information received from STAs and generate resourcescheduling information. The AP may generate resource schedulinginformation based on the SNR information on the subchannel or the SNRinformation on the tone group.

In operation 1430, the STA may receive the resource schedulinginformation from the AP. The AP may schedule frequency resources basedon information on candidate subchannels received from the plurality ofSTAs and generate resource scheduling information.

In operation 1440, the STA may identify a subchannel to be used forcommunications with the AP based on the resource scheduling informationand transmit data to the AP through the identified subchannel. The STAmay transmit the data to the AP at a transmit power level determined bythe AP.

Operations of the AP and the STAs not mentioned herein may refer torelevant descriptions of FIGS. 1 to 12.

The methods according to the embodiments may be realized as programinstructions implemented by various computers and be recorded innon-transitory computer-readable media. The media may also include,alone or in combination, the program instructions, data files, datastructures, and the like. The program instructions recorded in the mediamay be designed and configured specially for the embodiments or be knownand available to those skilled in computer software. Examples of thenon-transitory computer readable recording medium may include magneticmedia such as hard disks, floppy disks, and magnetic tape; optical mediasuch as CD ROM disks and DVDs; magneto-optical media such as flopticaldisks; and hardware devices that are specially configured to store andperform program instructions, such as read-only memory (ROM), randomaccess memory (RAM), flash memory, and the like. Examples of programinstructions include both machine codes, such as produced by a compiler,and higher level language codes that may be executed by the computerusing an interpreter. The described hardware devices may be configuredto act as one or more software modules in order to perform theoperations of the above-described exemplary embodiments, or vice versa.

While a few exemplary embodiments have been shown and described withreference to the accompanying drawings, it will be apparent to thoseskilled in the art that various modifications and variations can be madefrom the foregoing descriptions. For example, adequate effects may beachieved even if the foregoing processes and methods are carried out indifferent order than described above, and/or the aforementionedelements, such as systems, structures, devices, or circuits are combinedor coupled in different forms and modes than as described above or besubstituted or switched with other components or equivalents.

Thus, other implementations, alternative embodiments and equivalents tothe claimed subject matter are construed as being within the appendedclaims.

1. A wireless communication method performed by an access point (AP),the method comprising: performing channel sounding on a plurality ofsubchannels; identifying subchannels selected by stations among thesubchannels; scheduling communications between the AP and the stationsbased on the selected subchannels; and transmitting a data frame to thestations through the subchannels based on a scheduling result.
 2. Themethod of claim 1, wherein the scheduling of the communications betweenthe AP and the stations comprises scheduling a group of stations towhich the data frame is simultaneously transmitted through differentsubchannels based on the selected subchannels.
 3. The method of claim 2,wherein the transmitting of the data frame simultaneously transmits thedata frame to the stations through different subchannels based on thescheduling result.
 4. The method of claim 1, wherein the identifying ofthe subchannels selected by the stations identifies the subchannelsselected by the stations based on subchannels through which a frame istransmitted from the stations.
 5. The method of claim 1, wherein theperforming of the channel sounding comprises transmitting a soundingframe for channel sounding through the subchannels.
 6. The method ofclaim 5, wherein the transmitting of the sounding frame transmits a NullData Packet (NDP) frame through the subchannels within a time intervalof a Restricted Access Window (RAW) set by the AP, wherein the NDP frameis transmitted with a Long Training Field (LTF) padded so that NAPframes transmitted through the subchannels have the same length. 7.(canceled)
 8. The method of claim 5, wherein the stations performchannel estimation based on the NDP frame transmitted by the AP andselect a subchannel to use for communications among the plurality ofsubchannels based on a channel estimation result.
 9. The method of claim5, wherein the stations transmit a feedback on channel information andsubchannel selection information to the AP after receiving the soundingframe.
 10. (canceled)
 11. The method of claim 1, wherein the identifyingof the subchannels selected by the stations comprises receivingsubchannel selection information from the stations; and identifying thesubchannels selected by the stations based on the received subchannelselection information, and the subchannel selection informationcomprises selection information on at least one subchannel preferred bya station among the plurality of subchannels.
 12. The method of claim 1,wherein the stations transmit a frame to the AP through the selectedsubchannels among the plurality of subchannels.
 13. The method of claim1, wherein the stations transmit information on at least one or morepreferred subchannels among the plurality of subchannels to the AP. 14.The method of claim 1, wherein the transmitting of the data framecomprises identifying all multi-subchannels as one group identification(ID) to conduct signaling, wherein the transmitting of the data framesimultaneously transmits data frames to the stations through antennascorresponding to the respective subchannels based on a schedulingresult.
 15. The method of claim 1, wherein the transmitting of the dataframe comprises selecting a single-user multiple-input multiple-output(SU-MIMO) mode or multi-user multiple-input multiple-output (MU-MIMO)for each subchannel; and identifying a subchannel with the SU-MIMO modeselected as an association ID (AID) to conduct signaling and identifyinga subchannel with the MU-MIMO mode selected as a group ID to conductsignaling.
 16. The method of claim 1, wherein the scheduling of thecommunications between the AP and the stations comprises scheduling afrequency resource used for communications between the AP and thestations based on the subchannels selected by each station; andbroadcasting information on the scheduled frequency resource, whereinthe stations adjust a size of a feedback frame and transmit the feedbackframe with the size adjusted to the AP.
 17. (canceled)
 18. A wirelesscommunication method performed by a station, the method comprising:selecting a preferred subchannel among a plurality of subchannels;transmitting a frame to an access point (AP) through the selectedsubchannel; receiving resource scheduling information from the AP; andcommunicating with the AP based on the resource scheduling information.19. The method of claim 18, wherein the AP schedules a group of stationsto which a data frame is simultaneously transmitted through differentsubchannels based on subchannels selected by the stations, wherein theAP simultaneously transmits a data frame to stations through differentsubchannels based on a scheduling result.
 20. (canceled)
 21. The methodof claim 18, wherein the selecting of the subchannel comprisesperforming channel estimation based on a Null Data Packet (NDP) framereceived from the AP; and selecting the preferred subchannel among theplurality of subchannels based on a channel estimation result.
 22. Themethod of claim 21, wherein the NDP frame is transmitted through thesubchannels within a time interval of a Restricted Access Window (RAW)set by the AP.
 23. The method of claim 21, wherein the NDP frame istransmitted with a Long Training Field (LTF) padded so that NAP framestransmitted through the subchannels have the same length. 24-43.(canceled)
 44. An access point (AP) comprising: at least one antenna; areceiver to receive a frame through a subchannel among a plurality ofsubchannels from each of stations through the at least one antenna; anda transmitter to transmit resource scheduling information based onsubchannels selected by the stations to the stations through the atleast one antenna, wherein the subchannels elected by the stations areidentified by subchannels used to transmit the frame.
 45. (canceled)